Why does the sequential downloading option disappear after a certain time?

The option is only available for new torrents, if I'm downloading a series I want to enable it file by file so I can watch it before I've downloaded everything.

The only option I can see is to delete everything and start again! This time setting it at the start. Removing this option makes no sense what so ever!

It is always available. Go to Files tab, select files you'd like to download sequentially, right click > Priority > Sequential > On . Sequential downloading isn't optimal, Tixati doesn't download strictly piece after piece, but still your downloading may take longer than normally.